The invention relates to subscriber's line equipment for a telephone exchange comprising a two-wire-to-four wire conversion circuit, a circuit for supplying d.c. to the subscriber's line, and a closed loop detection circuit. The equipment can be supplemented in a known manner by a circuit for transmitting the ringing signal and a device for limiting the ringing current in the event of a shortcircuit on the subscriber's line.
In conventional subscriber's line equipment, use is normally made of a telephone transformer for two wire-to-four wire conversion, or sometimes use is made of a telephone frequency or voice frequency transformer (simpler and less bulky than a telephone transformer), a 48 V supply bridge, a current-limiting device and a closed-loop detecting means. Such equipment has the disadvantage of being bulky and expensive.
The equipment according to the invention does not have any of the aforementioned disadvantages since it does not comprise either a telephone transformer or a telephone-frequency transformer, but uses a ferrite-core transformer for higher frequencies. The ferrite-core transformer is less expensive and bulky and ensures complete galvanic decoupling of the subscriber's line, without any common point with the exchange supply source.
The equipment is of use more particularly in electromechanical or electronic telephone exchanges.
The equipment according to the invention is characterised in that it comprises:
A subscriber's line equipment for a telephone exchange, comprising:
A circuit for supplying direct current to the subscriber's line, which comprises: PA1 A step-up transformer having a ferrite core, with intermediate tapping of the primary winding, and primary winding terminals; PA1 Two pulse generators of rectangular pulses, each having a duty factor of 0.5 and a repetition radio frequency; PA1 A first PNP transistor being connected as a current generator, in series with said first current source, whereby: PA1 A third resistor, connected between the base of said first transistor and, by a fourth resistor, to the negative terminal of said second source; PA1 Two NPN transistors having a common emitter connector, whereby: PA1 rectifying means connected to the transformer secondary winding for supplying direct current to the subscriber's line; PA1 a first filtering capacitor shunting the subscriber's line; and PA1 a second high-frequency decoupling capacitor, connected between ground and said intermediate tapping of said transformer primary winding; and PA1 a first circuit for transmitting voice signals from the telephone exchange to the subscriber's line, having an impedance-matching, unit-gain buffer amplifier at its input connected by a fifth resistor to a current generator comprising a first differential amplifier having a negative-feedback loop connected to the base of said first transistor by a sixth resistor, and wherein said third resistor comprises a positive-feedback resistor; PA1 a second circuit for transmitting voice signals from the subscriber's line to said telephone exchange, comprising a low-pass filter having a seventh resistor and a third capacitor, wherein said seventh resistor is connected between the intermediate tapping of said transformer and the negative input of a second differential amplifier by an eighth resistor in series with a fourth capacitor, whereby said second differential amplifier has a negative feedback loop; PA1 a ninth resistor, for providing an anti-side tone effect, being connected between the output of said buffer amplifier and the connection between said fourth capacitor and said eighth resistor; and PA1 a tenth resistor, for matching the impedance relative to the telephone line, being connected between the output of said second differential amplifier and the negative input of said first differential amplifier.
First and second constant direct current sources, wherein: PA2 The negative terminal of said first source is connected to ground, and PA2 The positive terminal of said second source is connected to ground; PA2 The collector of said transistor is connected to the intermediate tapping of said transformer; PA2 The emitter of said transistor is connected, by a first resistor, to the positive terminal of said first source; PA2 The base of said transistor is connected, by a second resistor, to the positive terminal of said first source; PA2 The collectors of said transistors are respectively connected to said primary winding terminals of said transformer; PA2 The bases of said transistors are respectively connected to the output of the pulse generators; and PA2 the emitters of said transistors are connected to ground; and